1. Field of the Invention
The present invention relates to lift-out probes used in conjunction with a focused ion beam system. More specifically, the present invention relates to modifying an existing lift-out probe to provide an extension tip having a small tip diameter.
2. State of the Art
Focused ion beam (FIB) systems, which may include imaging capability using focused ion beam microscopy and scanning electron microscopy (SEM), are used extensively in failure analysis of semiconductor devices and for the preparation of electron-transparent specimens for transmission electron microscopy (TEM). The FIB lift-out technique is a commonly used procedure in which a small piece of sample is cut from a larger bulk sample, such as a silicon wafer containing integrated circuits, using a focused ion beam. After the sample has been ion milled, a lift-out probe, typically formed from a metal such as tungsten is contacted with the ion milled sample using a controller that is operably coupled to the FIB system. An organometallic gas flow is introduced through a tube that accesses the chamber that the sample is located in and an ion beam hits the organometallic gas at the interface between the lift-out probe and the sample to bond them together by deposition of a metal constituent of the gas. The ion milled sample may then be moved to another sample support for further analysis or processing by appropriate positioning of the lift-out probe with the sample attached thereto. The lift-out probe is then debonded from the sample by ion milling with the FIB. In the case of analysis using TEM, further ion milling to thin the sample to a suitable thickness is needed. Once the ion milled samples that have been removed from the larger samples have been thinned to an appropriate thickness, the sample may be analyzed using TEM.
The above process for removing samples using the FIB system and a lift-out probe is effective for preparing TEM samples. Commercially available lift-out probes, such as those available from Omniprobe, Inc., of Dallas, Tex., initially have a tip radius of as small as 0.1 μm and a 325 μm taper length. However, repeated use of the lift-out probes to contact samples and bond to samples, results in wear and damage to the probe tip. The lift-out probes are often worn prematurely to such an extent that the tip diameter is too large to effectively contact and be bonded to desirably small samples. This wear is partly due to the metal deposition onto the probe tip, but is also a result of having to cut a portion of the lift-out probe to sever it from the sample to which it is bonded with each use.
Once the tip diameter of a used lift-out probe is worn to around 10 μm, only larger samples can be lifted-out, or removed, from the larger bulk sample. This requirement subsequently results in longer ion milling times to produce TEM specimens. Furthermore, a larger lift-out probe is also more difficult to control and handle small specimens, such as TEM-ready samples of about 100 nm thickness. In addition, a larger, worn probe tip blocks the view of a location of interest on the larger bulk sample, and is more likely to cause damage to the sample.
Accordingly, there is a need for a practical technique to recondition used lift-out probes to provide small diameter probe tips suitable for effecting the lift-out of small ion milled specimens. Fulfilling this need would significantly reduce, if not eliminate, the need for replacement of the costly lift-out probes employed in FIB systems.